1. Field of the Invention
The present invention generally relates to an exhaust gas recirculating system, and more particularly to an exhaust gas recirculating system (EGR system) for purifying exhaust gas emitted from an exhaust system of an internal combustion engine by recirculating a part of the exhaust gas to an intake system.
2. Description of Related Art
The conventional exhaust gas recirculating system includes a recirculation control valve (EGR valve) on the way within an exhaust gas recirculation piping for recirculating a part of an exhaust gas emitted from an exhaust system to an intake system and regulates the recirculation gas flow rate by controlling the opening of the recirculation control valve according to the intake negative pressure, exhaust back pressure, etc. However, carbon component, etc. contained in the recirculation gas accumulates as a deposit on the valve element of the recirculation control valve and the inside wall of the exhaust gas recirculation piping. Particularly, within a deposit generation temperature range equal to or lower than approximately 110.degree. C., the deposit accumulation becomes large. If the deposit accumulates to an excessive level, even if the opening of the recirculation control valve is controlled to the target opening, the recirculation gas flow rate will not reach the target value or the closing ability of the recirculation control valve will degrade and gas leak through the valve when the recirculation control valve is closed will increase, exerting adverse effect on the engine horsepower, fuel efficiency and drivability.
In order to solve these problems, there has been disclosed a technique in the Japanese Unexamined Patent Publication No. 55-17678 in which a primary air introduction passage for introducing the primary air into an intake system is also used as the passage of the recirculation control valve and a speed reduction control valve is added to the recirculation control valve so that the deposit accumulated on the valve element of the recirculation control valve, etc. can be purged by the intake air pressure by the opening of the speed reduction control valve and the intake of the primary air to the intake side through the recirculation control valve when the engine is at deceleration.
However, according to the conventional construction in which the speed reduction control valve is added to the recirculation control valve and both valves are controlled, the construction is complicated and the manufacturing cost is high, and the demand for a compact construction can not be satisfied. Particularly, when it is so constructed that the exhaust gas is purified by the recirculation of a part of the exhaust gas even within the low water temperature range, the temperature of the recirculation gas flowing within the exhaust gas recirculation piping is low according to the conventional construction. As a result, the accumulation of the deposit increases so remarkably that the deposit can not sufficiently be removed by the intake air pressure alone.